Ableton Link is a new technology that synchronizes beat, phase and tempo of Ableton Live and Link-enabled iOS apps over a wireless network. It lets you play devices together with the freedom of a live band. Anyone can start and stop their part while others keep playing, and anyone can adjust the tempo and the rest will follow.
You can use Link to play with several instances of Ableton Live, with Live and iOS apps, or even without Live in your setup: using Link- enabled apps on multiple devices, or multiple apps on the same device."